User terminal and base station

ABSTRACT

A user terminal includes processor configured to execute processes of: receiving, from a base station, configuration information indicating whether the user terminal shall transmit a device-to-device (D2D) synchronization signal used for a D2D proximity service; transmitting the D2D synchronization signal in response to the received configuration information being first configuration information explicitly indicating the user terminal shall transmit the D2D synchronization signal; and prohibiting transmission of the D2D synchronization signal in response to the received configuration information being second configuration information explicitly indicating the user terminal shall not transmit the D2D synchronization signal.

RELATED APPLICATIONS

This application is a continuation application of internationalapplication PCT/JP2015/063262, filed May 8, 2015, which claims benefitof U.S. provisional application 61/990,951 filed May 9, 2014, theentirety of both applications hereby expressly incorporated byreference.

TECHNICAL FIELD

The present disclosure relates to a user terminal and a base stationwhich are used in a mobile communication system.

BACKGROUND ART

In 3GPP (3rd Generation Partnership Project) which is a project aimingto standardize a mobile communication system, the introduction of aDevice-to-Device (D2D) proximity service is discussed as a new functionon and after Release 12 (see Non Patent Document 1).

The D2D proximity service (D2D ProSe) is a service enabling directdevice-to-device communication within a synchronization clusterincluding a plurality of synchronized user terminals. The D2D proximityservice includes a discovery procedure (Discovery) in which a proximalterminal is discovered and D2D communication (Communication) that isdirect device-to-device communication.

Further, a discovery procedure in which a user terminal that exists in acertain cell (serving cell) discovers a proximal terminal that exists inother cell (neighbor cell) is called an inter-cell discovery procedure(Inter-Cell Discovery). Further, D2D communication in which a userterminal that exists in a serving cell performs communication with aproximal terminal that exists in a neighbor cell is called an inter-cellD2D communication (Inter-Cell Communication).

PRIOR ART DOCUMENTS Non Patent Document

-   [Non Patent Document 1] 3GPP technical report “TR 36.843 V12.0.1”    March, 2014

SUMMARY

A user terminal according to one embodiment includes processorconfigured to execute processes of: receiving, from a base station,configuration information indicating whether the user terminal shalltransmit a device-to-device (D2D) synchronization signal used for a D2Dproximity service; transmitting the D2D synchronization signal inresponse to the received configuration information being firstconfiguration information explicitly indicating the user terminal shalltransmit the D2D synchronization signal; and prohibiting transmission ofthe D2D synchronization signal in response to the received configurationinformation being second configuration information explicitly indicatingthe user terminal shall not transmit the D2D synchronization signal.

A user terminal according to one embodiment is a user terminal thatexists in a serving cell, in a mobile communication system that supportsa D2D proximity service. The user terminal includes: a receiverconfigured to receive inter-cell synchronization information on whethera neighbor cell is synchronized with the serving cell, from a basestation that forms the serving cell; and a controller configured todetermine, on the basis of the inter-cell synchronization information,whether to perform an inter-cell synchronization procedure forestablishing synchronization with a proximal terminal that exists in theneighbor cell, before performing inter-cell discovery procedure with theproximal terminal or inter-cell D2D communication with the proximalterminal.

A user terminal according to one embodiment exists in a serving cell, ina mobile communication system that supports a D2D proximity service. Theuser terminal includes: a controller configured to perform a process oftransmitting a D2D synchronization signal when the user terminalreceives, from a base station that forms the serving cell, informationthat instructs a transmission of the D2D synchronization signal.

A user terminal according to one embodiment is a user terminal thatexists in a serving cell, in a mobile communication system that supportsa D2D proximity service. The user terminal includes: a transmitterconfigured to transmit a D2D synchronization signal for establishingsynchronization with a proximal terminal that exists in a neighbor cell,before performing inter-cell discovery procedure with the proximalterminal or inter-cell D2D communication with the proximal terminal; anda controller configured to perform a control of cancelling transmissionof the D2D synchronization signal from the transmitter, on the basis ofdetection of a D2D synchronization signal transmitted from other userterminal that exists in the serving cell.

A base station according to one embodiment is a base station that formsa serving cell in which a user terminal exists, in a mobilecommunication system that supports a D2D proximity service. The basestation includes: a receiver configured to receive an inquiry as towhether it is possible to transmit a D2D synchronization signal forestablishing synchronization with a proximal terminal, from the userterminal; and a controller configured to determine whether to permittransmission of the D2D synchronization signal from the user terminal,on the basis of whether other user terminal that exists in the servingcell has been permitted to transmit the D2D synchronization signal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram of an LTE system according to a firstembodiment and a second embodiment.

FIG. 2 is a block diagram of a UE according to the first embodiment andthe second embodiment.

FIG. 3 is a block diagram of an eNB according to the first embodimentand the second embodiment.

FIG. 4 is a protocol stack diagram of a radio interface according to thefirst embodiment and the second embodiment.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem according to the first embodiment and the second embodiment.

FIG. 6 is a diagram illustrating an operation environment according tothe first embodiment.

FIG. 7 is a sequence diagram illustrating an operation according to thefirst embodiment.

FIG. 8 is a diagram illustrating an operation environment according tothe second embodiment.

FIG. 9 is a flowchart illustrating an operation of the UE according tothe second embodiment.

FIG. 10 is a sequence diagram illustrating an operation according to amodification of the second embodiment.

DESCRIPTION OF EMBODIMENTS Overview of Embodiment

A user terminal according to a first embodiment is a user terminal thatexists in a serving cell, in a mobile communication system that supportsa D2D proximity service. The user terminal includes: a receiverconfigured to receive inter-cell synchronization information on whethera neighbor cell is synchronized with the serving cell, from a basestation that forms the serving cell; and a controller configured todetermine on the basis of the inter-cell synchronization information,before performing an Inter-Cell Discovery with a proximal terminal thatexists in the neighbor cell or Inter-Cell D2D communication with theproximal terminal, whether to perform an inter-cell synchronizationprocedure for establishing synchronization with the proximal terminal.

In an operation pattern 1 according to the first embodiment, in theinter-cell synchronization procedure, the controller controls so that aD2D synchronization signal is transmitted in the serving cell.

In an operation pattern 2 according to the first embodiment, theinter-cell synchronization information includes information indicating asynchronization deviation amount between the serving cell and theneighbor cell.

In the operation pattern 2 according to the first embodiment, in theinter-cell synchronization procedure, the controller controls to besynchronized with the neighbor cell in accordance with thesynchronization deviation amount.

A user terminal according to a second embodiment is a user terminal thatexists in a serving cell, in a mobile communication system that supportsa D2D proximity service. The user terminal includes: a transmitterconfigured to transmit a D2D synchronization signal for establishing,before performing an Inter-Cell Discovery with a proximal terminal thatexists in a neighbor cell or Inter-Cell Communication with the proximalterminal, synchronization with the proximal terminal; and a controllerconfigured to control to cancel transmission of the D2D synchronizationsignal from the transmitter, on the basis of detection of the D2Dsynchronization signal transmitted from another user terminal thatexists in the serving cell.

In the second embodiment, when the controller detects a D2Dsynchronization signal transmitted from another user terminal thatexists in the serving cell and a reception level of the detected D2Dsynchronization signal exceeds a threshold value, the controllercontrols to cancel the transmission of the D2D synchronization signalfrom the transmitter.

A base station according to a modification of the second embodiment is abase station that forms a serving cell in which a user terminal exists,in a mobile communication system that supports a D2D proximity service.The base station includes: a receiver configured to receive an inquiryas to whether it is possible to transmit a D2D synchronization signalfor establishing synchronization with a proximal terminal, from the userterminal; and a controller configured to determine, on the basis ofwhether another user terminal that exists in the serving cell ispermitted to transmit the D2D synchronization signal, whether to permittransmission, by the user terminal, of the D2D synchronization signal.

In a modification of the second embodiment, when the controller permitsthe other user terminal to transmit the D2D synchronization signal andit is estimated that there is the other user terminal near the userterminal, the controller determines that the transmission, by the userterminal, of the D2D synchronization signal is not permitted.

First Embodiment

A case where the present disclosure is applied to an LTE system that isa mobile communication system based on 3GPP standards will be described,below.

(1) System Configuration

First of all, the configuration of an LTE system according to a firstembodiment will be described. FIG. 1 is a configuration diagram of theLTE system according to the first embodiment.

As illustrated in FIG. 1, the LTE system according to the firstembodiment includes UE (User Equipment) 100, E-UTRAN (Evolved-UMTSTerrestrial Radio Access Network) 10, and EPC (Evolved Packet Core) 20.

The UE 100 corresponds to a user terminal. The UE 100 is a mobilecommunication device, which performs radio communication with a cell (aserving cell). The configuration of the UE 100 will be described later.

The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10includes eNB 200 (an evolved Node-B). The eNB 200 corresponds to a basestation. The eNBs 200 are connected mutually via an X2 interface. Theconfiguration of the eNB 200 will be described later.

The eNB 200 manages one or a plurality of cells, and performs radiocommunication with the UE 100 that establishes a connection with a cellof the eNB 200. The eNB 200 has a radio resource management (RRM)function, a routing function of user data, a measurement controlfunction for mobility control and scheduling and the like. The “cell” isused as a term indicating a smallest unit of a radio communication area,and is also used as a term indicating a function of performing radiocommunication with the UE 100.

The EPC 20 corresponds to a core network. The EPC 20 includes MME(Mobility Management Entity)/S-GW (Serving-Gateway) 300. The MMEperforms different types of mobility control and the like for the UE100. The S-GW performs transfer control of the user data. The MME/S-GW300 is connected to the eNB 200 via an S1 interface. It is noted thatthe E-UTRAN 10 and the EPC 20 constitute a network of the LTE system.

FIG. 2 is a block diagram of the UE 100. As illustrated in FIG. 2, theUE 100 includes a plurality of antennas 101, a radio transceiver 110, auser interface 120, a GNSS (Global Navigation Satellite System) receiver130, a battery 140, a memory 150, and a processor 160. The memory 150and the processor 160 constitute a controller. The UE 100 may notnecessarily have the GNSS receiver 130. Furthermore, the memory 150 maybe integrally formed with the processor 160, and this set (that is, achip set) may be called a processor 160′.

The antenna 101 and the radio transceiver 110 are used to transmit andreceive a radio signal. The radio transceiver 110 converts a basebandsignal (a transmission signal) output from the processor 160 into aradio signal, and transmits the radio signal from the antenna 101.Furthermore, the radio transceiver 110 converts a radio signal receivedby the antenna 101 into a baseband signal (a reception signal), andoutputs the baseband signal to the processor 160.

The user interface 120 is an interface with a user carrying the UE 100,and includes, for example, a display, a microphone, a speaker, andvarious buttons. The user interface 120 receives an operation from auser and outputs a signal indicating the content of the operation to theprocessor 160. The GNSS receiver 130 receives a GNSS signal in order toobtain location information indicating a geographical location of the UE100, and outputs the received signal to the processor 160. The battery140 accumulates a power to be supplied to each block of the UE 100.

The memory 150 stores a program to be executed by the processor 160 andinformation to be used for processing by the processor 160. Theprocessor 160 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signal,and a CPU (Central Processing Unit) that performs various types ofprocesses by executing the program stored in the memory 150. Theprocessor 160 may further include a codec that performs encoding anddecoding on sound and video signals. The processor 160 executes varioustypes of processes and various types of communication protocolsdescribed later.

FIG. 3 is a block diagram of the eNB 200. As illustrated in FIG. 3, theeNB 200 includes a plurality of antennas 201, a radio transceiver 210, anetwork interface 220, a memory 230, and a processor 240. The memory 230and the processor 240 configure a controller. Furthermore, the memory230 may be integrally formed with the processor 240, and this set (thatis, a chipset) may be called a processor.

The antenna 201 and the radio transceiver 210 are used to transmit andreceive a radio signal. The radio transceiver 210 converts a basebandsignal (a transmission signal) output from the processor 240 into aradio signal, and transmits the radio signal from the antenna 201.Furthermore, the radio transceiver 210 converts a radio signal receivedby the antenna 201 into a baseband signal (a reception signal), andoutputs the baseband signal to the processor 240.

The network interface 220 is connected to the neighboring eNB 200 viathe X2 interface and is connected to the MME/S-GW 300 via the S1interface. The network interface 220 is used in communication performedon the X2 interface and communication performed on the S1 interface.

The memory 230 stores a program to be executed by the processor 240 andinformation to be used for processing by the processor 240. Theprocessor 240 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signaland a CPU that performs various types of processes by executing theprogram stored in the memory 230. The processor 240 executes varioustypes of processes and various types of communication protocolsdescribed later.

FIG. 4 is a protocol stack diagram of a radio interface in the LTEsystem. As illustrated in FIG. 4, the radio interface protocol isclassified into a first layer to a third layer of an OSI referencemodel, such that the first layer is a physical (PHY) layer. The secondlayer includes a MAC (Medium Access Control) layer, an RLC (Radio LinkControl) layer, and a PDCP (Packet Data Convergence Protocol) layer. Thethird layer includes an RRC (Radio Resource Control) layer.

The physical layer performs encoding and decoding, modulation anddemodulation, antenna mapping and demapping, and resource mapping anddemapping. Between the physical layer of the UE 100 and the physicallayer of the eNB 200, user data and control signals are transmitted viaa physical channel.

The MAC layer performs priority control of data, a retransmissionprocess by a hybrid ARQ (HARQ), a random access procedure, and the like.Between the MAC layer of the UE 100 and the MAC layer of the eNB 200,user data and control signals are transmitted via a transport channel.The MAC layer of the eNB 200 includes a scheduler for determining atransport format (a transport block size and a modulation and codingscheme) of an uplink and a downlink, and resource blocks to be assignedto the UE 100.

The RLC layer transmits data to an RLC layer of a reception side byusing the functions of the MAC layer and the physical layer. Between theRLC layer of the UE 100 and the RLC layer of the eNB 200, user data andcontrol signals are transmitted via a logical channel.

The PDCP layer performs header compression and decompression, andencryption and decryption.

The RRC layer is defined only in a control plane that handles controlsignals. Between the RRC layer of the UE 100 and the RRC layer of theeNB 200, a control signal (an RRC message) for various types of settingsis transmitted. The RRC layer controls a logical channel, a transportchannel, and a physical channel according to the establishment,re-establishment, and release of a radio bearer. When there is aconnection (an RRC connection) between the RRC of the UE 100 and the RRCof the eNB 200, the UE 100 is in an RRC connected state. Otherwise, theUE 100 is in an RRC idle state.

An NAS (Non-Access Stratum) layer positioned above the RRC layerperforms session management, mobility management and the like.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem. In the LTE system, OFDMA (Orthogonal Frequency Division MultipleAccess) is applied to a downlink, and SC-FDMA (Single Carrier FrequencyDivision Multiple Access) is applied to an uplink, respectively.

As illustrated in FIG. 5, a radio frame is configured by 10 subframesarranged in a time direction. Each subframe is configured by two slotsarranged in the time direction. Each subframe has a length of 1 ms andeach slot has a length of 0.5 ms. Each subframe includes a plurality ofresource blocks (RBs) in a frequency direction, and a plurality ofsymbols in the time direction. Each resource block includes a pluralityof subcarriers in the frequency direction. One symbol and one subcarrierform a one resource element. Of the radio resources (time and frequencyresources) assigned to the UE 100, a frequency resource can beidentified by a resource block and a time resource can be identified bya subframe (or a slot).

(2) D2D Proximity Service

A D2D proximity service will be described, below. An LTE systemaccording to the first embodiment supports the D2D proximity service.

The D2D proximity service (D2D ProSe) is a service enabling directUE-to-UE communication within a synchronization cluster including aplurality of synchronized UEs 100. The D2D proximity service includes adiscovery procedure (Discovery) in which a proximal UE is discovered andD2D communication (Communication) that is direct UE-to-UE communication.The D2D communication is also called Direct communication.

A scenario in which all the UEs 100 forming the synchronization clusterare located inside a cell coverage is called “In coverage”. A scenarioin which all the UEs 100 forming the synchronization cluster are locatedoutside a cell coverage is called “Out of coverage”. A scenario in whichsome UEs 100 in the synchronization cluster are located inside a cellcoverage and the remaining UEs 100 are located outside the cell coverageis called “Partial coverage”.

In “In coverage”, the eNB 200 is a D2D synchronization source. A D2Dnon-synchronization source, from which a D2D synchronization signal isnot transmitted, is synchronized with the D2D synchronization source.The eNB 200 that is a D2D synchronization source transmits, by abroadcast signal, D2D resource information indicating radio resources(resource pool) available for the D2D proximity service. The D2Dresource information includes information indicating a resource pool forthe discovery procedure (Discovery resource information) and informationindicating a resource pool for the D2D communication (Communicationresource information), for example. The UE 100 that is a D2Dnon-synchronization source performs the discovery procedure and the D2Dcommunication on the basis of the D2D resource information received fromthe eNB 200.

In “Out of coverage” or “Partial coverage”, the UE 100 is a D2Dsynchronization source. In “Out of coverage”, the UE 100 that is a D2Dsynchronization source transmits D2D resource information indicatingradio resources (resource pool) available for the D2D proximity service,by a D2D synchronization signal, for example. The D2D synchronizationsignal is a signal transmitted in the synchronization procedure in whicha device-to-device synchronization is established. The D2Dsynchronization signal includes a D2D SS and a physical D2Dsynchronization channel (PD2DSCH). The D2D SS is a signal for providinga synchronization standard of a time and a frequency. The PD2DSCH is aphysical channel through which a greater amount of information can beconveyed than the D2D SS can. The PD2DSCH conveys the above-describedD2D resource information (the Discovery resource information and theCommunication resource information). Alternatively, when the D2D SS isassociated with the D2D resource information, the PD2DSCH may berendered unnecessary.

The discovery procedure is used mainly when the D2D communication isperformed by unicast. When starting the D2D communication with anotherUE 100, one UE 100 uses any particular radio resource in the resourcepool for the discovery procedure to transmit the Discovery signal. Whenstarting the D2D communication with the one UE 100, the other UE 100scans the Discovery signal within the resource pool for the discoveryprocedure to receive the Discovery signal. The Discovery signal mayinclude information indicating radio resources used by the one UE 100for the D2D communication.

Further, a discovery procedure in which a user terminal that exists in acertain cell (serving cell) discovers a proximal terminal that exists inanother cell (neighbor cell) is called an inter-cell discovery procedure(Inter-Cell Discovery). Further, D2D communication in which a userterminal that exists in a serving cell performs communication with aproximal terminal that exists in a neighbor cell is called inter-cellD2D communication (Inter-Cell Communication).

(3) Operation Environment

An operation environment according to the first embodiment will bedescribed, below. FIG. 6 is a diagram illustrating an operationenvironment according to the first embodiment.

As illustrated in FIG. 6, an eNB 200#1 forms a cell #1, and an eNB 200#2forms a cell #2. The cell #1 and the cell #2 are in a relationship wherethe both are adjacent to each other.

The UE 100#1 exists in the cell #1. The UE 100#1 is in a RRC connectedstate or a RRC idle state in the cell #1. When the UE 100#1 isconcerned, the cell #1 is a serving cell and the cell #2 is a neighborcell.

The UE 100#2 exists in the cell #2. The UE 100#2 is in a RRC connectedstate or a RRC idle state in the cell #2. When the UE 100#2 isconcerned, the cell #1 is a neighbor cell and the cell #2 is a servingcell.

In the first embodiment, in such an operation environment, a scenario isassumed where the UE 100#1 performs the Inter-Cell Discovery with the UE100#2. Further, a scenario is assumed where the cell #2 is notsynchronized with the cell #1. When the cell #2 is not synchronized withthe cell #1, the UE 100#1 is non-synchronized with the UE 100#2, andthus, it is highly probable that the Inter-Cell Discovery with the UE100#2 is failed even when the UE 100#1 tries.

(4) Operation According to First Embodiment

An operation according to the first embodiment will be described, below.

(4.1) Operation Overview

FIG. 7 is a sequence diagram illustrating an operation according to thefirst embodiment.

As illustrated in FIG. 7, in step S101, the eNB 200#1 transmits theinter-cell synchronization information on whether the cell #2 (neighborcell) is synchronized with the cell #1 (serving cell). The UE 100#1receives the inter-cell synchronization information from the eNB 200#1.

The inter-cell synchronization information is transmitted by a broadcastsignal. Alternatively, the inter-cell synchronization information may betransmitted by a unicast signal. The inter-cell synchronizationinformation preferably includes an identifier of the cell #2. Theinter-cell synchronization information may be included in the samemessage as the D2D resource information.

In step S102, the UE 100#1 determines, on the basis of the inter-cellsynchronization information, whether to perform an inter-cellsynchronization procedure (Inter-Cell Synchronization) to establish thesynchronization with the UE 100#2 (proximal terminal). Specifically, theUE 100#1 determines that the Inter-Cell Synchronization is unnecessarywhen the cell #2 is synchronized with the cell #1. On the other hand,the UE 100#1 determines that the Inter-Cell Synchronization is performedwhen the cell #2 is not synchronized with the cell #1.

When the cell #2 is not synchronized with the cell #1 (step S102: NO),the Inter-Cell Synchronization is performed in step S103. There are twopatterns for the Inter-Cell Synchronization. Each operation pattern willbe described in detail later.

In step S104, the UE 100#1 performs the Inter-Cell Discovery with the UE100#2.

Thus, the UE 100#1 receives the inter-cell synchronization informationfrom the eNB 200#1 forming the cell #1 (serving cell). The UE 100#1determines whether to perform the Inter-Cell Synchronization forestablishing the synchronization with the UE 100#2 on the basis of theinter-cell synchronization information, before performing the Inter-CellDiscovery with the UE 100#2 (proximal terminal) that exists in the cell#2 (neighbor cell).

Thus, the UE 100#1 is capable of performing the Inter-CellSynchronization after confirming that the cell #2 is not synchronizedwith the cell #1. Therefore, it is possible to appropriately perform theInter-Cell Discovery after establishing the synchronization with the UE100#2.

On the other hand, when the cell #2 is synchronized with the cell #1,the UE 100#1 is capable of omitting the Inter-Cell Synchronization.Therefore, it is possible to restrain an increase in process load,interference, etc., caused as a result of performing an unnecessaryInter-Cell Synchronization.

(4.2) Operation Pattern 1

Next, an operation pattern 1 of the Inter-Cell Synchronization will bedescribed.

In the operation pattern 1, in the Inter-Cell Synchronization, the UE100#1 transmits a D2D SS (D2D synchronization signal). The UE 100#2 thatreceives the D2D SS is capable of being synchronized with the UE 100#1.Thus, the synchronization between the UE 100#1 and the UE 100#2 isestablished.

The UE 100#2 that receives the D2D SS is capable of being synchronizedwith the UE 100#1. Thus, the synchronization between the UE 100#1 andthe UE 100#2 is directly established.

Alternatively, in the Inter-Cell Synchronization, the UE 100#1 may scanthe D2D SS transmitted from the UE 100#2. As a result of the scan, theUE 100#1 may start transmitting the D2D SS when the D2D SS transmittedfrom the UE 100#2 is not detected. Note that it is assumed that the D2DSS includes information indicating a serving cell of a UE from which theD2D SS is transmitted.

As described above, the D2D SS is used in a case of the Out of coverageor the Partial coverage; however, in the first embodiment, the D2D SS isexchanged in a case of the In coverage.

In the operation pattern 1, the inter-cell synchronization informationtransmitted from the eNB 200#1 may include a 1-bit flag indicatingwhether the cell #2 is synchronized with the cell #1. In the inter-cellsynchronization information, the flag is associated with the identifierof the cell #2.

The UE 100#1 starts transmitting (or scanning) the D2D SS, determiningthat the Inter-Cell Synchronization is performed, when the flagindicates that the cell #2 is not synchronized with the cell #1.

(4.3) Operation Pattern 2

Next, an operation pattern 2 of the Inter-Cell Synchronization will bedescribed.

In the operation pattern 2, the inter-cell synchronization informationtransmitted from the eNB 200#1 is information indicating asynchronization deviation amount between the cell #1 (serving cell) andthe cell #2 (neighbor cell). The information indicating thesynchronization deviation amount includes a radio frame offset value ofthe cell #2 relative to the cell #1, a subframe offset value of the cell#2 relative to the cell #1, etc.

In the operation pattern 2, the eNB 200#1 may transmit the inter-cellsynchronization information on the cell #2, only when the cell #2 is notsynchronized with the cell #1. That is, then eNB 200#1 may not transmitthe inter-cell synchronization information on the cell #2, when the cell#2 is synchronized with the cell #1. In this case, the UE 100#1determines that the Inter-Cell Synchronization is performed whenreceiving the inter-cell synchronization information on the cell #2 fromthe eNB 200#1.

Alternatively, the eNB 200#1 may transmit the inter-cell synchronizationinformation on the cell #2 also when the cell #2 is synchronized withthe cell #1. In this case, the offset value indicating thesynchronization deviation amount is set to zero. In this case, the UE100#1 determines that the Inter-Cell Synchronization is performed whenreceiving the inter-cell synchronization information including an offsetvalue greater than zero.

In the operation pattern 2, in the Inter-Cell Synchronization, the UE100#1 controls to be synchronized with the cell #2 in accordance withthe synchronization deviation amount. For example, adjustment of atransmission timing of the Discovery signal and/or adjustment of areception timing of the Discovery signal are performed. As a result ofthe UE 100#1 controlling to be synchronized with the cell #2 inaccordance with the synchronization deviation amount, synchronizationbetween the UE 100#1 and the UE 100#2 is established.

Second Embodiment

A difference of the second embodiment from the first embodiment will bemainly described. In the second embodiment, a case is assumed where theoperation pattern 1 of the above-described Inter-Cell Synchronization isapplied.

In the operation pattern 1 of the above-described Inter-CellSynchronization, each UE 100 that performs the Inter-CellSynchronization exchanges the D2D SS in “In coverage”, and thus, it maybe possible that interference increase resulting from the D2D SS. Thus,in the second embodiment, in the operation pattern 1 of theabove-described Inter-Cell Synchronization, an increase in interferenceis restrained by restraining the transmission of the D2D SS.

(1) Operation Environment

An operation environment according to the second embodiment will bedescribed, below. FIG. 8 is a diagram illustrating the operationenvironment according to the second embodiment.

As illustrated in FIG. 8, the UE 100#1 and the UE 100#3 exist in acoverage of the cell #1. Further, the UE 100#2 exists in a coverage ofthe cell #2. Each of the UE 100#1 to the UE 100#3 is a UE 100 thatperforms the Inter-Cell Synchronization to perform the Inter-CellDiscovery.

The UE 100#3 is located near the UE 100#1 and transmits a D2D SS. Asdescribed above, the D2D SS includes information indicating a servingcell of the transmission source.

Each of the UE 100-1 and the UE 100-2 receives the D2D SS from the UE100#3. The UE 100#2 establishes the synchronization with the UE 100#3 byusing the D2D SS in response to reception of the D2D SS from the UE100#3 on the neighbor cell.

Here, the UE 100#1 and the UE 100#3 that exist in the identical cell aresynchronized with each other, and thus, this means that the UE 100#2establishes the synchronization with the UE 100#1 simultaneously ofestablishing the synchronization with the UE 100#3. Thus, the UE 100#1does not need to transmit the D2D SS.

(2) Operation According the Second Embodiment

An operation according to the second embodiment will be described,below.

The UE 100#1 according to the second embodiment scans the D2D SStransmitted from the UE 100#3 (another UE) that exists in the cell #1(serving cell), before performing the Inter-Cell Discovery with the UE100#2 (proximal terminal) that exists in the cell #2 (neighbor cell).

Then, the UE 100#1 cancels transmission of the D2D SS from the UE 100#1on the basis of detection of the D2D SS transmitted from the UE 100#3that exists in the cell #1. Here, when the UE 100#1 detects the D2D SStransmitted from the UE 100#3 that exists in the cell #1 and a receptionlevel of the detected D2D SS exceeds a threshold value, the UE 100#1preferably cancels transmission of the D2D SS from the UE 100#1.

FIG. 9 is a flowchart illustrating an operation of the UE 100#1according to the second embodiment. For example, the UE 100#1 starts thepresent flow when determining that the Inter-Cell Discovery isperformed.

As illustrated in FIG. 9, in step S201, the UE 100#1 scans the D2D SS tomeasure the reception level of the received D2D SS.

In step S202, the UE 100#1 determines whether the serving cellidentifier included in the D2D SS matches the cell identifier of thecell #1. When matching, the UE 100#1 determines whether the receptionlevel of the D2D SS exceeds a threshold value.

When “NO” in step S202, in step S203, the UE 100#1 starts (or continues)transmitting the D2D SS. On the other hand, when “YES” in step S202, instep S204, the UE 100#1 cancels transmitting the D2D SS.

Thus, in the inter-Cell Synchronization, the UE 100#1 that exists in thecell #1 cancels transmitting the D2D SS from the UE 100#1 on the basisof detection of the D2D SS transmitted from the other UE that exists inthe cell #1. Thus, it is possible to restrain transmission of the D2DSS.

Modification of Second Embodiment

In the above-described second embodiment, the UE takes the initiative tocontrol the transmission of the D2D SS. However, the eNB may take theinitiative to control the transmission of the D2D SS. In a modificationof the second embodiment, a case is assumed where it is not possible totransmit the D2D SS if it is not possible to obtain permission from theeNB 200. However, a basic concept is similar to that of the secondembodiment.

An operation according to the modification of the second embodiment willbe described, below. Here, an operation in an operation environment asillustrated in FIG. 8 will be described.

FIG. 10 is a sequence diagram illustrating an operation according to themodification of the second embodiment. In FIG. 10, the UE 100#1 and theUE 100#3 exist in the cell #1 formed by the eNB 200#1.

As illustrated in FIG. 10, in step S301, the UE 100#3 transmits, to theeNB 200#1, an inquiry as to whether it is possible to transmit the D2DSS. The inquiry may include geological location information of the UE100#3. The eNB 200#1 determines whether to permit transmission of theD2D SS, on the inquiry from the UE 100#3. Here, description proceedswith an assumption where permission of the transmission of the D2D SS isgiven to the UE 100#3. The eNB 200#1 stores information on the UE 100#3in which the transmission of the D2D SS is permitted.

In step S302, the eNB 200#1 notifies the UE 100#3 of transmissionpermission of the D2D SS. The UE 100#3 starts transmitting the D2D SSonce the transmission of the D2D SS is permitted.

In step S303, the UE 100#1 transmits, to the eNB 200#1, an inquiry as towhether it is possible to transmit the D2D SS. The inquiry may includegeological location information on the UE 100#1.

In step S304, the eNB 200#1 determines whether to permit transmission ofthe D2D SS, on the inquiry from the UE 100#1. The eNB 200#1 alreadypermits the UE 100#3 to transmit the D2D SS, that is, the UE 100#3 istransmitting the D2D SS, and thus, the eNB 200#1 may determine that thetransmission by the UE 100#1 of the D2D SS is not permitted.

Alternatively, when the permission of the transmission of the D2D SS isalready given to the UE 100#3 and it is estimated that there is the UE100#3 near the UE 100#1, the eNB 200#1 may determine that thetransmission, by the UE 100#1, of the D2D SS is not permitted. In thiscase, the eNB 200#1 may determine on the basis of the geologicallocation information included in the inquiry whether there is the UE100#3 near the UE 100#1. Alternatively, when the eNB 200#1 manages apathloss value of each UE 100, an uplink transmission power value, or atiming advance (TA) value, the eNB 200#1 may make a determination on thebasis of the pathloss value, the uplink transmission power value, or theTA value, instead of the geological location information.

Here, description proceeds with an assumption where permission oftransmission of the D2D SS is not given to the UE 100#1.

In step S305, the eNB 200#1 notifies the UE 100#1 of prohibition oftransmission (denial) of the D2D SS. The UE 100#1 does not starttransmitting the D2D SS because the transmission of the D2D SS is notpermitted.

Thus, according to the modification of the second embodiment, similarlyto the above-described second embodiment, it is possible to restraintransmission of the D2D SS.

It is noted that in the modification of the second embodiment, in orderto manage a transmission status of the D2D SS by the eNB 200#1, it ispreferable that the UE 100 that stops transmitting the D2D SS inquiresor notifies the eNB 200#1 to that effect.

Other Embodiments

In above-described each embodiment, a scenario is assumed where the UE100#1 performs the Inter-Cell Discovery with the UE 100#2. However, thepresent disclosure may be also applied to a scenario where the UE 100#1performs the Inter-Cell Communication with the UE 100#2 while the UE100#1 does not perform the Inter-Cell Discovery with the UE 100#2. Thatis, the “Inter-Cell Discovery” in the operation according toabove-described each embodiment can be replaced by the “Inter-CellCommunication”.

Furthermore, in the embodiment described above, although an LTE systemis described as an example of a mobile communication system, the presentdisclosure is not limited to the LTE system, and may be applied to asystem other than the LTE system.

[Additional Statements]

Below, supplementary notes of the embodiments will be noted.

INTRODUCTION

As for the inter-cell discovery, a following agreement on D2D receptiondiscovery resource pools is made.

The eNB may provide D2D reception discovery resources in SIB. These maycover resources used for D2D transmission in this cell as well asresources used in neighbour cells. (Details FFS)

On the other hand, RAN1 made a following agreement.

Confirm that a radio resource pool(s) may be provided by eNB for D2D UEsin SIB for discovery reception for Type-2B (if supported). FFS whetherthe common reception pool(s) or different reception pools for type 1 andType-2B discovery. UE is not required to decode neighbor cell SIB.

Mechanisms for Type-2B discovery. A resource hopping mechanism followingthe resource allocation by eNB can be applied. FFS details of resourcehopping mechanism.

In this additional statements, we will investigate other aspects ofinter-cell discovery and propose possible solutions, based on the aboveagreements.

DISCUSSION

Regarding the synchronization among serving cell and neighbouring cells,the following 2 deployments have been considered until last RAN2meeting.

Synchronous Deployment

For synchronous deployment, serving cell is synchronized withneighbouring cells, so that D2D UEs can perform inter-cell discoverytransmission/reception by referring the synchronization signaltransmitted from serving cell. Therefore, serving cell is not requiredto transmit additional information other than reception resource pools.Synchronous deployment can be achieved by OAM and eNB implementation.For example, synchronous deployments may be assumed within each MBSFNarea.

Asynchronous Deployment

For the asynchronous deployment, serving cell is not synchronized withneighbouring cells, so that D2D UE in serving cell is required to besynchronized with D2D UE in neighbouring cell before performinginter-cell discovery transmission/reception.

As for the inter-cell discovery, both types of deployment are assumed,therefore, inter-cell discovery should also be considered for both typesof deployment. For reducing the complexity, it is considered that commonscheme to perform inter-cell discovery should be introduced for bothsynchronous and asynchronous deployment. It is assumed the scheme can beclassified based on whether or not D2D UEs perform directsynchronization with other D2D UEs for inter-cell discovery.

(Direct Synchronization with Other D2D UEs: Alt.1)

With direct synchronization, the in-coverage D2D UE transmits D2DSS orreceives D2DSS transmitted by D2D UE in the neighbouring cell beforeperforming inter-cell discovery in order to be synchronized with D2D UEin the neighbouring cell. Since this scheme assumes directsynchronization with other D2D UEs, it does not matter whether or notthe neighbouring cell is synchronized with the serving cell; therefore,this scheme is applicable for both synchronous and asynchronousdeployments.

To achieve direct synchronization efficiently, in-coverage D2D UEtransmits D2DSS before transmitting discovery signal to D2DUE inneighbouring cell. Similarly, in-coverage D2D UE monitors D2DSStransmitted by D2D UE in neighbouring cell before receiving discoverysignal from D2D UE in neighbouring cell. One of the drawbacks of thisscheme is the increased interference among D2DSS transmissions within NWcoverage. Therefore, further enhancement is needed to minimize thenumber of D2D UEs transmitting D2DSS for inter-cell discovery.

(Non-Direct Synchronization with Other D2D UEs: Alt.2)

With this scheme, the serving cell should inform the in-coverage D2D UEof the neighbouring cell's timing in order to perform inter-celldiscovery on the asynchronous deployment, so that the in-coverage D2D UEcan perform inter-cell discovery transmission/reception withouttransmitting/receiving D2DSS irrespective of whether or not theneighbouring cell is synchronized with the serving cell. It is assumedthat neighbouring cell's timing can be provided to the serving cell byOAM as well as neighbouring cell's reception resource pools. Therefore,serving cell should provide this timing information to D2D UEs as wellas neighbour cells' reception resource pools.

Based on the descriptions of both alternatives above, RAN2 shouldconsider which alternative should be adopted for inter-cell D2Ddiscovery.

Proposal 1: RAN2 should discuss which alternative should be adopted forinter-cell D2D discovery.

Proposal 2: If Alt.2 is agreed, the serving cell should provide timinginformation as well as reception resource pools.

Clearly, other modifications and manners of practicing this inventionwill occur readily to those of ordinary skill in the art in view ofthese teachings. The above description is illustrative and notrestrictive. This invention is to be limited only by the followingclaims, which include all such modifications and manners of practicewhen viewed in conjunction with the above specification and accompanyingdrawings. The scope of the invention should, therefore, be determinednot with reference to the above description, but instead should bedetermined with reference to the appended claims along with their fullscope of equivalents.

INDUSTRIAL APPLICABILITY

The present disclosure is useful for communication fields.

1. A user terminal comprising: processor configured to execute processesof receiving, from a base station, configuration information indicatingwhether the user terminal shall transmit a device-to-device (D2D)synchronization signal used for a D2D proximity service, transmittingthe D2D synchronization signal in response to the received configurationinformation being first configuration information explicitly indicatingthe user terminal shall transmit the D2D synchronization signal, andprohibiting transmission of the D2D synchronization signal in responseto the received configuration information being second configurationinformation explicitly indicating the user terminal shall not transmitthe D2D synchronization signal.
 2. The user terminal according to claim1, wherein the processor executes a process of transmitting the D2Dsynchronization signal in response to the user terminal receiving thefirst configuration information, wherein the user terminal transmits aD2D discovery signal used for discovering proximal user terminals. 3.The user terminal according to claim 1, wherein the processor executes aprocess of transmitting the D2D synchronization signal in response to areceived power of a signal received by the user terminal being less thana threshold value even if the user terminal does not receive the firstconfiguration information.
 4. A base station comprising: processorconfigured to execute processes of transmitting, to a first userterminal, first configuration information that explicitly indicates thefirst user terminal shall transmit a device-to-device (D2D)synchronization signal used for a D2D proximity service, andtransmitting, to a second user terminal, second configurationinformation that explicitly indicates the second user terminal shall nottransmit the D2D synchronization signal.
 5. A device for controlling auser terminal, comprising: processor configured to execute processes ofreceiving, from a base station, configuration information indicatingwhether the user terminal shall transmit a device-to-device (D2D)synchronization signal used for a D2D proximity service, transmittingthe D2D synchronization signal in response to the received configurationinformation being first configuration information explicitly indicatingthe user terminal shall transmit the D2D synchronization signal, andprohibiting transmission of the D2D synchronization signal in responseto the received configuration information being second configurationinformation explicitly indicating the user terminal shall not transmitthe D2D synchronization signal.
 6. The device according to claim 5,wherein the processor executes a process of transmitting the D2Dsynchronization signal in response to the user terminal receiving thefirst configuration information, wherein the user terminal transmits aD2D discovery signal used for discovering proximal user terminals. 7.The device according to claim 5, wherein the processor executes aprocess of transmitting the D2D synchronization signal in response to areceived power of a signal received by the user terminal being less thana threshold value even if the user terminal does not receive the firstconfiguration information.